The present invention relates to an apparatus for the manufacture of corrugated paperboard and, more particularly, to an adjustable ballast system for a double facer apparatus.
In a typical prior art double racer, a single web is brought into contact with the glued flute tips of a single face corrugated web and the freshly glued double face web is passed over the surfaces of a number of serially arranged hot plates to cause the starch-based glue to set. Double face web travel over the hot plates is provided by a wide driven holddown belt in direct contact with the upper face of the corrugated web, and the outer face of the belt is held in contact with the web by a series of ballast rollers or the like, all in a well known manner.
For a number of reasons, it is desirable to provide some means for varying the vertical ballast load imposed on the holddown belt, both longitudinally in the direction of belt and web movement through the double facer and laterally across the width of the belt and web. As the three paper web components comprising a double face corrugated web vary in quality and/or web thickness from one run of corrugated board to another or as double or triple wall board consisting of 2 or 3 single face webs and a bottom liner, more or less heat may be required to be applied in the double facer and, as a result, more or less ballast weight applied to the holddown belt. Variations in the speed at which the web is run may also require variations in ballast in the longitudinal direction of the web. Lateral variations in the holddown ballast across the width of the web may be desirable to compensate for the actual web width being run or because of variations in the moisture content of web components across the width of the web. Furthermore, combinations of the foregoing factors may make it desirable to simultaneously vary the holddown ballast loading both longitudinally and laterally in the double facer.
U.S. Pat. Nos. 3,676,268 and 3,753,838 both disclose systems for controlling the weight of ballast rollers positioned over the holddown belt in a double facer. The earlier patent provides rows of individual rollers extending laterally of the web (or in the cross machine direction) with several rows of rollers commonly mounted over an individual heating plate for movement together into and out of contact with the holddown belt. In addition, each roller in the group is mounted on its own vertical fluid actuator for individual positioning to vary the load imposed by the roller on the belt. The later patent utilizes parallel beams which are spaced across the width of the belt, with each beam carrying a row of rollers spaced in the direction of web movement and extending over the surfaces of a number of adjacent heating plates. Each roller is individually mounted and spring biased against the belt and each beam is independently adjustable to vary the vertical position of each row of rollers.
Other arrangements for applying variable ballast pressure to the holddown belt in a double facer are shown, for example, in U.S. Pat. No. 3,319,353 in which air pressure from overhead plenums holds the belt against the face of the corrugated web; in U.S. Pat. No. 3,607,523 in which full web width ballast rolls are lifted off the holddown belt automatically as the hot plates or steam chests pivot down and out of contact with the underside of the corrugated web; in U.S. Pat. No. 5,256,240 which utilizes pressurized air bladders to compensate for surface distortion in the supporting hot plates; and, in U.S. Pat. No. 4,049,485 where ballast roll pressure is controlled and uniformly varied along the whole heating section by a common linear actuator operating against the force of adjustable bias springs.
Although driven holddown belts are conventional and have long been used in double backers, the holddown belt and the system required to mount and drive it add considerably to the cost of the system, as well as to the complexity of operation. The typical holddown belt, which is driven to circulate around upstream and downstream pulleys, absorbs large amounts of heat transmitted through the corrugated paperboard web from the underlying hot plate system. The pulleys are typically large diameter so that the ballast load system can be positioned between the operating and return runs of the holddown belt. As a result, the holddown belt system also takes up significant additional space and dissipates large amounts of heat into the plant environment on the return run of the belt traveling in the open air above the ballast load system. The presence of a holddown belt atop the web also inhibits substantially the ability to dissipate heat rapidly, such as for emergency shutdown or web production changes.
It would be most desirable, therefore, if the holddown belt system in a double backer could be eliminated and yet retain the capability of providing a controlled web holddown force to assure proper curing of the adhesive and uniform drying of the web.